This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to integrated circuit design.
Typically, one of the greatest expenses in fabricating an integrated circuit is the cost of the labor required to manufacture and package the device. Thus, while costs are none-the-less impacted by the loss of an integrated circuit early on in processing, such losses represent more the loss of anticipated revenue alone. However, the loss of a device near or at the end of the processing represents not only the loss of anticipated revenue, but the loss of actual labor costs that were incurred to process the device to the point at which it failed. This is particularly compounded by the fact that after the integrated circuits are diced and separated one from another, the labor required to complete a packaged device tends to be more individually devoted to a single device, thereby amplifying the labor costs that were previously divided between all integrated circuits on a single substrate. Further, the packaging for a device tends to have a significant material cost associated with it. Therefore, failures of packaged devices tend to be the costliest of all.
Ensuring that devices survive the fabrication process and are functional and reliable is one of the primary goals of failure analysis. Failure analysis attempts to analyze a failed device, including both the integrated circuit and the package, to determine the cause of failure and then feed back the information to the appropriate source so that procedures can be instituted to prevent future failures of the same type. Unfortunately, accomplishing effective failure analysis is often a difficult thing to do.
For example, failures may be cause by a great variety of problems, and it often is not clear which factors are contributing to the failures. As a more specific example, it is often difficult to determine whether a failure is a design flaw, a processing flaw, an execution flaw, or a combination of these flaws. A design flaw is one in which the process produces the device as designed, and the process was executed correctly, but the design of the device itself is not reliable. A processing flaw is one in which the fundamental design of the device is sound, and the process was executed correctly, but the process is not capable to reliably produce the device. Finally, an execution flaw is one in which the design and the process are both sound, but the process was not executed properly.
Unfortunately, it is very difficult to determine which of the problems is contributing to a given failure, especially when the failure may be sporadic, and more especially when different components of all three of the primary factors described above may be contributing to the failure. For example, it has been observed that the electrically conductive traces on a substrate, such as a circuit board to which circuits are mounted, tend to crack near the comers and edges of the substrate. A failure such as this could possibly have its roots in any one or more of design flaws, processing flaws, or execution flaws.
What is needed, therefore, is a solution to the problem of electrically conductive traces that crack in the comers of packaged devices.
The above and other needs are met by a substrate of the type for receiving an integrated circuit and a mold cover. The mold cover covers a first portion of the substrate and leaves a second portion of the substrate exposed with a boundary edge between the first portion of the substrate and a second portion of the substrate. The substrate has electrically conductive traces and electrically conductive vias on an upper layer adjacent the mold cover. The electrically conductive traces do not cross the boundary edge on the upper layer of the substrate.
Thus, it has been advantageously discovered that by having no electrically conductive traces that underlie the edges of the mold cover, the incidence of cracked electrically conductive traces in the comers and edges of the substrate is greatly reduced. In this manner the yield of packaged devices is improved.
In a preferred embodiment of the invention, the boundary edge, across which no electrically conductive traces extend, extends completely around the peripheral edges of the substrate. According to other aspects of the invention there are provided a packaged device that includes the substrate as described above, a method of fabricating a packaged device, and a method of designing a substrate.